1. Field of the Invention
The present invention pertains to a method of manufacturing engine parts, and more particularly to a method of finishing a connecting rod of a piston engine.
2. Description of the Related Art
Connecting rods for piston engines, such as the internal combustion engine of an automobile, transfer power from the pistons to the crank shaft. Typically manufacturers of connecting rods first form the rough shape of the connecting rod by either forging or casting methods. They must then machine the rough connecting rods to obtain the finished product. The specific method of machining the rough connecting rod to obtain the finished product can greatly affect the cost and quality of the finished connecting rod.
A connecting rod generally has a small hole at one end and a large hole at the opposing end. (For example, see FIGS. 1 and 2.) FIGS. 3A-8 provide an illustration of prior art methods of finishing connecting rods. Typically, the first step in finishing the small-hole end is to apply a milling process to the inner surface of the small hole such that it conforms to the desired tolerances. The bold lines in FIG. 3A represent this first step. In the second step, one chamfers the edge as indicated by bolds lines in FIG. 3B. In the third step, one mills the inner surface of the small hole a second time to remove burs that the chamfering step generated, thus allowing the small hole to be used as a reference hole (see FIG. 4). In the fourth step, one cuts the workpiece along parallel planes such that they may serve as reference planes. The bold lines in FIG. 5 represent the cross-sectional view of these parallel plane surfaces. Next, one uses the reference surface and reference planes of the small hole to simultaneously process both the small and large hole to satisfy combined tolerances. This is indicated schematically by bold lines in FIG. 6. In this last step, one processes surfaces of the large hole (see FIG. 1) such that they coincide with the reference planes.
If one can process the connecting rod such that the width at the small-hole end (B.sub.1 in FIG. 2) is smaller than the width at the large-hole end (B.sub.2 in FIG. 2), one can obtain additional design flexibility and other advantages such as weight reduction. To achieve this, many of the prior methods involved cutting the parallel planes deeper than that of the fourth step as indicated in bold lines in FIGS. 7A and 7B. One would then follow this cutting step by a chamfering step.
There are several problems and limitations of such prior art methods of producing connecting rods. Typically, one cannot maintain a steady production flow of connecting rods of both the type in which B.sub.1 is equal to B.sub.2, and the type in which B.sub.1 is less than B.sub.2. In addition, the chamfering step leads to the formation of burs, as one can see illustrated in FIG. 8. Finally, the cutting action imposes stresses on the connecting rod that tend to distort the small-hole region from the desired specifications. This puts limitations on how small one can make B.sub.1.